Lecture 11
Global Positioning System (GPS)
          GPS is network of satellites (24 total - 21 in use, 3 spares) that continuously transmit coded information, which makes it possible to precisely identify locations on earth by measuring distance from the satellites. GPS provides continuous (round the clock), real time, 3-dimensional positioning, navigation and timing worldwide in any weather condition. It was originally intended for military applications, but in the 19805 it was made available for civilian use. There are no subscription fees or setup charges to use GPS. Any person With a GPS receiver can access the system, and it can be used for any application that requires location coordinates.
Components of GPS
          The GPS system consists of three segments: 1) the segments: the GPS satellites themselves, 2) the control system, and 3) the user segment, which includes both military and civilian users and their CPS equipment. The space segment is composed of the GPS satellites that transmit signals from space based on which time and position of the user is measured. The whole set of satellite is called ‘constallation’ and also known as Global Navigation satellite system (GNSS). There are three globally known GNSS. These are NAVSTAR (NAVigation System using Timing and Ranging) of USA, GLONASS (Globalnaya Navigazionnaya Sputnikovaya Sistema, or Global Navigation satellite System) of Russia, and GALILEO of European Union. Indian Regional Navigation Satellite System (IRNSS) is an independent regional navigation satellite system being developed by India. It is designed to provide accurate position information service to user in India as well as in the region extending up to 1500 km from its boundary, which is its primary service area. The control segment consists of Monitor Stations, Ground Antennas and a Master Control station (MCS). The monitor stations passively track all satellites in view, accumulating ranging data. This information is processed at the MCS to determine satellite orbits and to update each satellite’s navigation message Updated information is transmitted to each satellite via the Ground Antennas. The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert satellite signal into position, velocity, and time estimates. Four satellites are required to compute the four dimensions X, Y, Z (position) and time. GPS receivers are used for navigation, positioning, time dissemination, and other research. There are civilian applications for GPS in almost every field, from surveying to transportation to natural resource management to agriculture. Most civilian uses of GPS, however, fall into one of four categories: navigation, surveying, mapping and timing.

 Working Principles of GPS
          Each satellite transmits a message containing three pieces of information: the satellite number; its position in space, and the time at which the message was sent. The GPS receiver reads the message and saves the information. GPS receivers take this information and use triangulation (otherwise called trilateration) to calculate the user’s exact location.
          A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in View, the receiver can determine the user’s 3D position unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

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